Exposure control device

ABSTRACT

A light sensitive resistor for measuring light value of a scene to be photographed is in a bridge circuit from which the unbalance signal operates switches either to light lamps for guiding manual adjustment of exposure factors or to control a servomotor that makes the necessary adjustments. These adjustments, whether automatic or manual, produce changes in the bridge elements, so that balance of the bridge is reached when the proper exposure factor settings are made. Control of the servomotor and/or indication lamps is accomplished by means of an astable multivibrator of which the effective polarity of the output is changed by changing the duty cycle. The exposure factor adjustment obtained by balancing the bridge is stored in the value of a variable resistor which, after the shutter is released, controls, for example, the exposure time.

United States Patent [1 1 Takishima Oct. 16, 1973 EXPOSURE CONTROLDEVICE [75] Inventor:

[22] Filed: May 25, 1972 [21] Appl. No.: 256,998

Yoshiyuki Takishima, Tokyo, Japan [30] Foreign Application Priority DataMay 28, 1971 Japan 46/36799 May 28, 1971 Japan 46/36800 [52] US. Cl95/10 CE, 95/53 EB [51] Int. Cl. G03b 7/08 [58] Field of Search 95/10 C,10 CE, 10 CT, 95/53 EA, 53 EB, 64 R, 64 B, 64 C, 64 D [5 6] ReferencesCited UNITED STATES PATENTS 3,695,158 10/1972 Fahlenberg 95/10 CE3,581,643 6/1971 Yoshimura 95/10 CE X 3,452,656 7/1969 Ruhle et al.95/10 CE 3,426,662 2/1969 Sevin 95/10 CE 3,670,634 6/1972 Kessler et a1.95/10 CT X 3,503,313 3/1970 Kuramoto 95/53 EB X 3,625,124 12/1971Miyakawa 95/10 CT 3,593,629 7/1971 Rentschler 95/10 CE 3,602,717 8/1971Konig 95/10 CT X FOREIGN PATENTS OR APPLICATIONS 10,625 5/1969 Japan95/10 CT Primary ExaminerJoseph F. Peters, Jr. Attorney-William R.Woodward [5 7] ABSTRACT A light sensitive resistor for measuring lightvalue of a scene to be photographed is in a bridge circuit from whichthe unbalance signal operates switches either to light lamps for guidingmanual adjustment of exposure factors or to control a servomotor thatmakes the necessary adjustments. These adjustments, whether automatic ormanual, produce changes in the bridge elements, so that balance of thebridge is reached when the proper exposure factor settings are made.Control of the servomotor and/or indication lamps is accomplished bymeans of an astable multivibrator of which the effective polarity of theoutput is changed by changing the duty cycle. The exposure factoradjustment obtained by balancing the bridge is stored in the value of avariable resistor which, after the shutter is released, controls, forexample, the exposure time.

10 Claims, 4 Drawing Figures inn SHEET 1 OF 2 PAIENIEDIIN 16 mm FIG.

FIG. 3

FIG. 4

PATENTEUHBHB ma 3.765312 SHEET 2 BF 2 BACKGROUND OF THE INVENTION Thepresent invention relates to a photoelectric exposure control deviceespecially suitable for compact cameras.

Automatic of manual exposure control devices are known for use withstill cameras or cine cameras in which a photoconductive orphotoelectric element is used to measure the light intensity of asubject. For example in an automatic exposure control device, aservomotor may be driven in response to the output of a photoelectriccell so that an optimum aperture or shutter speed may be set. Moreparticularly the photoelectric cell constitutes one arm of a bridgecircuit, and an aperture setting device which is disposed in frontof'the photoelectric cell and is drivingly coupled to the servomotor isactuated and stopped when the bridge circuit is balanced, the servomotoris stopped. In this way, the optimum aperture may be set.

SUMMARY OF THE INVENTION One of the objects of the present invention isto provide an exposure control device in which in addition to a bridgecircuit including a photoelectric cell as one arm, means is provided forobtaining a desired shutter speed or aperture, and an exposure factorcontrol circuit is adjusted automatically by a servomotor or manually byreference to a lamp indication of the balance or unbalance of when thebridge circuit.

In the present invention an exposure control device is provided in whichprior to the shutter release the resistance of a photoelectric cell isstored in a resistor which constitutes one arm of a balanced bridgecircuit, and in response to this stored resistance, the shutter speed iscontrolled. The electronic circuit is symmetrical and voltagestabilizing or regulation circuits are provided. A motor isintermittently driven. The exposure control device is reliable inoperation with a high degree of accuracy.

Another feature of the present invention is an exposure control devicein which the output of a bridge circuit, which is used as a photometriccircuit, is detected by a differential type detector circuit and inresponse to the two outputs of said detector circuit two switchingcircuits are energized. In response to the outputs of the switchingcircuits, an astable multivibrator starts and stops oscillating withoutputs of variable duty cycle. In one embodiment the balance orunbalance of the bridge circuit is indicated by lamps, whereby theoptimum aperture or shutter speed may be manually set.

According to one aspect of the present invention, prior to the shutterrelease, an exposure factor such as aperture or shutter speed isdetermined in response to the light intensity of the subject and isstored so that the exposure is made in response to this stored exposurefactor. The present invention is best suited for use in the so-calledexposure control device capable of storing an exposure factor which isused in a single-lens reflex camera. In the so-called TTL camera inwhich a photoelectric cell is disposed for example in the proximity ofan eyepiece in order to receive the light passing through a camera lens,the intensity of light which reaches a film may be measured with ahigher degree of accuracy. However, when the mirror is pulled away fromthe light beam which falls on the film, no light is intercepted by thephotoelectric cell. Therefore prior to the shutter release, the lightintensity must be stored so that an aperture or shutter speed may be setto an optimum value in response to the stored data. The exposure controldevice in accordance with the present invention is best suited for usewith the TTL single-lens reflex camera of the type described, but itshould be understood that the present invention may be also applied toother cameras of the type in which the light intensity of a subject ismeasured and stored before the shutter release is pressed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of a firstembodiment of an exposure control device in accordance with the presentinvention;

FIG. 2 is a practical electronic circuit diagram thereof;

FIG. 3 is a block diagram of a second embodiment of the presentinvention; and

FIG. 4 is a practical electronic circuit diagram thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, thefirst embodiment of the present invention which is shown as beingapplied to a single-lens reflex camera will be described. The light froma subject passing through a camera lens L is reflected by a mirror M anda penta prism P, and intercepted by a photoelectric cell 1, which isshown as being a CdS cell in the instant embodiment and constitutes onearm of a bridge circuit 2. A variable resistor R in an electronicshutter speed control circuit 7 constitutes one arm of the bridgecircuit 2 and is controlled by a d-c servomotor 6 as will be describedin more detail hereinafter. Variable resistors which constitute the twoarms of the bridge circuit 2 are set depending upon the exposure factorsas will be described in more detail hereinafter. In response to the twooutputs from a differential amplifier type detector circuit 3, twoswitching circuits 4 and 4 are actuated, and in response to the outputsof the switching circuits 4 and 4' an astable multivibrator 5 starts andstops oscillating at definite times with a desired waveform. In responseto the outputs from the astable multivibrator 5, the d-c servomotor 6rotates so as to vary the resistance of the variable resistor R whichalso constitutes a time constant circuit with a capacitor C in theelectronic shutter speed control circuit 7 and which is operativelycoupled to the d-c servomotor 6 as indicated by the dashed line 6". Theshutter speed control circuit 7 further includes a switching circuitsuch as a Schmitt circuit which makes transistions between two stablestates in response to the polarity of the voltage applied across thecapacitor C. An electromagnet 8 actuates a shutter mechanism (notshown), and a display circuit 9 incorporates a flash lamp whichindicates the balanced or unbalanced condition of the bridge circuit 2.

Next the mode of operation of the first embodiment will be described.The film speed or sensitivity and an aperture or stop are set in thebridge circuit 2, and the light from the subject is intercepted by thephotoelectric cell 1. The unbalanced bridge 2 is detected by thedifferential type detector circuit 3 so that in response to the polarityof the output from the bridge circuit 2 the detector circuit 3 actuateseither of the switching circuits 4 or 4. For example, the switchingcircuit 4 is ON whereas the switching circuit 4' is OFF. The astablemultivibrator starts oscillating and the d-c servomotor 6 starts torotate so that the resistance of the variable resistor R in one arm ofthe bridge circuit 2 is varied to balance the bridge circuit 2. When thebridge circuit 2 is balanced, the output of the detector circuit 3becomes zero so that both the switching circuits 4 and 4 are turned OFF.As a result the astable multivibrator 5 stops oscillating so that thed-c servomotor 6 also stops. Hence the time constant of the timeconstant circuit R and C is set in response to the resistance of thephotoelectric cell 1, because the resistance of the variable resistor Rhas been varied to balance the bridge circuit 2. Therefore, in responseto this time constant the shutter speed is determined to obtain theoptimum exposure. In summary, according to the present invention, theresistance of the photoelectric cell 1 before the shutter is actuated isstored as the resistance of the variable resistor in one arm of thebridge circuit and also of the shutter speed control circuit. Since thelamp in the display circuit 9 flashed to indicate whether the bridgecircuit 2 is balanced or not balanced, an alarm signal indicating thatthe light incident upon the photoelectric cell 1 is too bright or toodark may be obtained.

Next referring to FIG. 2, the practical electric circuit of the firstembodiment described so far with reference to FIG. 1 will be describedin detail. In the circuit, a photoelectric cell 10 which corresponds tothat designated by 1 in FIG. 1, a variable resistor 11 operativelycoupled to a d-c servomotor 52 and two variable resistors 12 and 13constitute a bridge circuit. The resistances of the variable resistors12 and 13 are varied in response to the exposure factors suchas the filmspeed or ASA and a desired stop which in turn are set for example by afilm speed setting ring and an aperture setting ring respectively. Thedifferential type detector circuit comprises six transistors 14-19, anda constant current circuit comprises two transistors 20 and 21 anddiodes 22, 23 and 24. The power is supplied from batteries 26 through anON-OFF power switch 25.

The two switching circuits comprise two sets of transistors 27-33 and34-40, and the reference voltages at which the two switching circuitsare actuated are obtained by varying the emitter voltages applied totransistors 41 and 42 which constitute a constant current circuit. Theastable multivibrator comprises two transistors 43 and 44, and the timeconstant circuit comprises resistors 47, 48 and 49 and capacitors 50 and51. The display circuit comprises transistors 45, 46, 60 and 61, a flashlamp 62, and a time constant circuit comprising a resistor 58 and acapacitor 59. A voltage stabilizing circuit comprises a transistor 63and diodes 64. The time constant circuit in the electronic shutter speedcontrol circuit comprises a resistor 53 which is operatively coupled tothe d-c servomotor 52 and a capacitor 54. It should be noted thatinstead of the resistor 53, the resistor 1 1 in the bridge circuit maybe used, but when the resistor 53 is used, the so-called gammacorrection may become possible because the change in gamma which is aslow function of B in an expression given by where 1,, photocurrent of aphotoelectric cell;

C constant; and B light intensity may be compensated by varying theresistance of the variable resistor 53 per unit displacement of thesliding arm thereof.

The shutter speed control circuit denoted by 5 in FIG. 1 furthercomprises two transistors 55 and 56, and an electromagnet 57 whichcorresponds to that denoted by 8 in FIG. 1 actuates the shutter blindsof the conventional shutter mechanism not shown A selection switch 66for actuating the shutter mechanism is operatively coupled to a shutterrelease button or the like not shown Next the mode of operation will bedescribed. First the power switch 25 is closed and the film speed and adesired aperture or stop are set in the variable resistors 12 and 13. Inthe TTL camera, the light from the subject passing through the cameralens is intercepted by the photoelectric cell 10 while the apertureblades not shown are maintained in the wide opened position exposuresetting at maximum aperture The output of the unbalanced bridge circuitis detected by the differential type detector circuit 2 whose outputs inturn are applied to the transistors 30 and 37 of the input stages of thetwo switching circuits respectively and compared with the referencevoltages. When the input voltage to the transistor 30 is lower than thereference voltage, the transistors 30-33 in one switching circuit areconductive whereas the transistors 34-40 in the other switching circuitare turned off. The output from the output stage of the one switchingcircuit comprising the transistors 32 and 33 is applied to the baseelectrodes of the transistors 43 and 44 in the astable multivibrator sothat the latter starts oscillating. In this case, the duty cycle of theoscillating waveforms of the output from the astable multivibrator isdetermined depending upon the resistors K47, R and R in the timeconstant circuit when the capacitances of the capacitors C and C areequal. Therefore, the outputs with the opposite polarities are derivedfrom the astable multivibrator and applied to the d-c servomotor 52.That is, the positive and negative outputs with the pulse-like waveformsare applied to the d-c servomotor 52 so that the latter rotates in onedirection which corresponds to the positive or negative output having alarger pulse width and the negative or positive output retards therotation of the d-c servomotor 52. In response to the angle of rotationof the d-c servomotor 52, the resistance of the variable resistor 11 inthe bridge circuit is varied so as to balance the latter. When thebridge circuit is balanced, the inputs to the differential type detectorcircuit have the same level so that the two switching circuits areturned off. As a result, no voltage is applied to the bases of thetransistors 43 and 44 in the astable multivibrator so that the latterstops oscillating. Hence, the condition for attaining the optimumexposure, that is the resistance which the variable resistor 11 musthave in order to balance the bridge circuit is now stored in theresistor 53 in the shutter speed control circuit which is operativelycoupled to the d-c servomotor 52. Therefore, when the first shutterblind starts to travel to open the shutter in response to the shutterrelease operation, the movable contact of the selection switch 66 isswitched from the stationary contact a to the contact b so that thecapacitor 54 is charged through the resistor 53. Therefore in responseto the time constant determined by thecapa'citor 54 and the resistor 53,the transistor 55 in the Schmitt trigger circuit is made conductingwhereas the transistor 56 is turned off so that the electromagnet 57 isde-energized to cause the second rolled blind to travel a predeterminedtime after the first rolled blined has started to travel. Hence theshutter is now closed.

When either of the switching circuits is turned on, the transistors 45and 46 in the display circuit are made conducting and the transistors 60and 61 are then turned on at a predetermined time depending upon thetime constant of the CR circuit 59 and 58 after the transistors 45 and46 have been made conducting. As a result, the lamp 62 is turned on. Thelamp 62 is turned off when the bridge circuit is balanced because thetransistors in the display circuit are all turned off.

So far the mode of operation has been described in case of the exposuresetting with maximum aperture, but in case of exposure setting withstopping down of the maximum F-number of the camera lens used, theappropriate correction is made by changing the aperture setting resistorin the bridge circuit by the aperture setting mechanism not shown whichis actuated by the servomotor.

As described above, according to the present invention, the resistanceof the photoelectric cell in the bridge circuit is stored in theresistor 53 by the servomotor 52 and in response to the shutter releaseoperation the resistor 53 is switched to the time constant circuit inthe shutter speed control circuit. Hence the light intensity of thesubject before the shutter is released may be converted into theresistance and is stored in the resistor 53 so that the exposure settingmay be accomplished with a higher degree of accuracy. The firstembodimentdescribed with reference to FIG. 2 is arranged almost entirelywith differential type transistor circuits so that the operation willnot be adversely affected by temperature variation and variation insupply voltage can be prevented from producing erratic operation.Furthermore, the circuit arrangement is almost symmetrical so that thecircuit of the first embodiment may be formed as an integrated circuit.As this circuit may be fabricated in extremely compact form, it is wellsuited for incorporation in a miniature camera.

Next referring to FIG. 3, the second embodiment of the present inventionwill be described. A photoelectric cell 101 from which is derived theelectrical output representing the light intensity of the subject isinserted as one arm of a bridge circuit 102. In response to the outputsof the bridge circuit 102, a differential type detector circuit 103actuates switching circuits 104 and 104 which in turn actuates anastable multivibrator 105. Either of two lamps in a display circuit 106is turned on in order to indicate that the bridge circuit is unbalanced,and is turned off indicating the balanced bridge circuit. The lightpassing through the camera lens L is intercepted by the photoelectriccell 101. In the figure, a shutter blind disposed in front of a film Fis denoted by Sh, and an aperture setting device D is operativelycoupled to a variable resistor R A variable resistor R, is operativelycoupled to a shutter speed setting dial and to a variable resistor Rwhich constitutes a time constant circuit with a capacitor C. Themovable contact of a selection switch Sw is switched from the fixedcontact a to the contact b in response to the shutter release operation.An electromagnet EM operatively coupled to a Schmitt trigger circuit 8'actuates the shutter blind Sh.

In operation, the resistance of the variable resistor R, which isoperatively coupled to the shutter speed setting dial or ring and whichconstitutes one of the arms of the bridge circuit 102 is set dependingupon a shutter speed or film sensitivity, and the light from the subjectpassing through the camera lens L is intercepted by the photoelectirccell 101. The output of the bridge circuit which is not balanced isdetected by the differential type detector circuit 103 so that inresponse to the output of the detector circuit 103 either of theswitching circuits 104 or 104 is actuated. In response to the output ofthe switching circuit 104 or 104, the astable multivibrator 105 startsoscillating. The duty cycle of the output of the astable multivibrator105 changes depending upon which of the two switching circuits 104 and104 is energized. The output of the astable multivibrator 105 is appliedacross the capacitor in the display circuit 106 so that either of thelamps may be turned on depending upon the voltage difference across thecapacitor in order to indicate that the bridge circuit is unbalanced.Next the variable resistor in one arm of the bridge circuit which isoperatively coupled to the aperture setting device D is so adjusted thatthe bridge circuit 102 may be balanced. When the bridge circuit 102 isbalanced, the lamps in the display circuit are all turned off. Hence,the optimum aperture is obtained. In case of the exposure setting atstopping down, the aperture setting ring on the side of theinterchangeable lens may be adjusted instead of the variable resistor Rso as to adjust the light incident upon the photoelectric cell 101thereby balancing the bridge circuit 102. Alternatively, a desiredaperture may be set by the adjustment of the variable resistor R and there sistance of the variable resistor R, which is coupled to the shutterspeed setting dial or the like may be varied in order to attain thebalance of the bridge circuit, that is the optimum exposure setting.When the bridge circuit 102 is balanced and the lamps in the displaycircuit are turned off, the resistance of the variable resistor R isalso set to a desired value. Therefore, when the shutter button or thelike is depressed, the shutter mechanism is actuated depending upon thetime constant of the time constant circuit R and C as in the case of thefirst embodiment. Hence the shutter speed is controlled.

Next referring to FIG. 4 illustrating the practical electric circuit ofthe second embodiment, the latter will be described in more detail.Since the arrangement of the photo-electric cell 101, the bridge circuit102, the differential type detector circuit 103, and the switchingcircuits 104 and 104 are similar to those shown in FIG. 2, they are notshown for simplicity and only the astable multivibrator 105 and thedisplay circuit 106 are shown in FIG. 4 and will be described in detail.

The astable multivibrator 105 comprises two transistors 143 and 144,resistors 147, 148 and 149 and capacitors 150 and 151. The period of theastable multivibrator 105 is determined by the resistors 147-149 and thecapacitors 150 and 151. A capacitor 201 in the display circuit 106 is ofnon-polar type. Reference numerals 202 and 203 denote the lamps; 204 and205, diodes; 206, atransistor in a constant current circuit of thedisplay circuit 106; and 207, diodes for supplying a constant voltage.

Next the mode of operation will be described. In response to the lightintensity, the resistance of the photo-electric cell 101 varies. Theresistance of the variable resistor R is varied depending upon the filmsensitivity or shutter speed. The output of the unbalanced bridgecircuit 102 is applied through the differential type detector circuit103 to the bases of the transistors 143 and 144 in the astablemultivibrator 105 so that the latter starts oscillating as in the caseof the first embodiment. In this case, the duty cycle of the oscillatingoutput of the astable multivibrator 105 is determined depending uponwhich of the two switching circuits 104 and 104 is energized. That is,when the capacitances of the capacitors 150 and 151 are equal, the dutycycle is a function of the ratio of the resistor R to theseries-connected resistors R and R when the switching circuit 140 isenergized or of the ratio of the resistor R to the series-connectedresistors R and R when the switching circuit 104' is energized. Thepotential difference between the collectors of the transistors 150 and151 in the astable multivibrator 105 is determined by the above ratio ofthe resistance when the capacitors C and C are equal in value and whenthe resistors R and R are also equal in value. Depending upon thispotential difference the current flows through either of the diode 204or 205 so that the lamp 203 or 204 is turned on. The lamps 202 and 203are marked with the arrows which serve to indicate the direction ofrotation of the aperture or shutter speed setting ring. That is, theaperture setting ring is rotated in the direction indicated by the arrowmarked on the turned-on lamp 202 or 203 so as to vary the resistance ofthe variable resistor R, in the bridge circuit 102 to balance thelatter. Then, the output of the bridge circuit 102 becomes zero so thatthe transistors in the switching circuits 104 and 104' are all turnedoff. As a result, no voltage is applied to the bases of the transistors143 and 144 in the astable multivibrator 105 so that the latter stopsoscillating. Therefore, both lamps 202 and 203 in the display circuit106 are turned off,

indicating that the aperture and the shutter speed are set for optimumexposure. In case of the exposure setting at stopping down, that is incase of balancing the bridge circuit 102 by controlling the lightincident upon the photo-electric cell 101 by the adjustment of theaperture setting ring, the correction for the maximum F- number of thecamera lens used may be made by the adjustment of the variable resistorin the bridge circuit 102.

Instead of the lamps 202 and 203 in the display circuit 106, photodiodesmay be used, and instead of the two lamps, only one lamp may beconnected between the collectors of the transistors 143 and 144 of theastable multivibrator 105 in order to indicate the balanced orunbalanced bridge circuit 102.

As described above, in the second embodiment of the electronic shutterin accordance with the present invention, the astable multivibratorstarts and stops oscillating with the output waveforms of variable dutycycle in response to the outputs of the switching circuits, and thelamps are used to indicate'the balanced or unbalanced bridge circuit sothat the second embodiment is especially suited to be fabricated as anintegrated circuit and that the voltage stabilization may be attained.

Hence, the shutter release operation may be accomplished with a higherdegree of accuracy.

I claim:

1. In an exposure control device controllable in response to the outputof a photoelectric cell, an improvement comprising the combination of:

a. light quantity control means,

b. an astable multivibrator having two output termi-- nals,

c. a second control means interconnected between said two outputterminals of said astable multivibrator for controlling said lightquantity control means,

d. switching means comprising at least one switching circuit which isactuated in response to the output of said photoelectric cell, saidswitching means being electrically coupled to said astable multivibratorfor controlling the intervals at which said astable multivibrator makestransistions, and

e. said light quantity control means including storage means which iscontrolled in response to the output of said second control means forstoring an exposure factor as the state of a variable exposure factorcontrol element prior to the shutter release operation and alsoincluding means for causing said element to control an exposure factorafter shutter release.

2. An improvement as set forth in claim 1 wherein said astablemultivibrator further includes a pair of time constant circuits whosetime constants during multivibrator operation are different from eachother and mutually interchangeable by operation of said switching means.

3. An improvement as set forth in claim 1 wherein said switching meansincludes a bridge circuit in one arm of which said photoelectric cell islocated and the output terminals of which are electrically coupled tothe input terminals of said switching means; and wherein said switchingmeans is made conductive in response to the output of said bridgecircuit when unbalanced and then causes said astable multivibrator tostart oscillating.

4. An improvement as set forth in claim 1 wherein said second controlmeans comprises a motor which is drivingly coupled to said variableexposure factor element.

5. An improvement as set forth in claim 1 wherein said light quantitycontrol means has manual controls and said second control meanscomprises display means for guiding the use of said manual controls.

6. An improvement as set forth in claim 3 wherein said switching meanscomprises a pair of switching circuits which are differential amplifiercircuits.

7. An improvement as set forth in claim 2 wherein said time constantcircuits have a common resistor .network and wherein said switchingmeans comprises a bridge circuit one arm of which comprises saidphotoelectric cell, and a pair of switching circuits to which areapplied the output of said bridge circuit, and wherein said switchingcircuits are coupled to said resistor network of said time constantcircuits so as to complete said time constant circuits at one of twopoints, or not at all, according to the state of said switching means,such point of completion being such as to determine the sense of thedisparity of said time constant.

8. An improvement as set forth in claim 3 wherein said astablemultivibrator comprises two transistors and a pair of time constantcircuits whose capacitors are coupled to a series of resistorsdifferentially and unequally apportionable to said respective capacitorsby selection through said switching means of the circuit return point insaid series of resistors.

shutter speed control means which comprises a CR time constant circuitcomprising said variable resistor and a capacitor, a detector circuitfor detecting a voltage drop across said capacitor, and electromagnetmeans coupled to said detector circuit for closing a shutter when saidvoltage drop has reached a predetermined value, said motor beingoperatively coupled to said variable resistor for varying itsresistance.

1. In an exposure control device controllable in response to the outputof a photoelectric cell, an improvement comprising the combination of:a. light quantity control means, b. an astable multivibrator having twooutput terminals, c. a second control means interconnected between saidtwo output terminals of said astable multivibrator for controlling saidlight quantity control means, d. switching means comprising at least oneswitching circuit which is actuated in response to the output of saidphotoelectric cell, said switching means being electrically coupled tosaid astable multivibrator for controlling the intervals at which saidastable multivibrator makes transistions, and e. said light quantitycontrol means including storage means which is controlled in response tothe output of said second control means for storing an exposure factoras the state of a variable exposure factor control element prior to theshutter release operation and also including means for causing saidelement to control an exposure factor after shutter release.
 2. Animprovement as set forth in claim 1 wherein said astable multivibratorfurther includes a pair of time constant circuits whose time constantsduring multivibrator operation are different from each other andmutually interchangeable by operation of said switching means.
 3. Animprovement as set forth in claim 1 wherein said switching meansincludes a bridge circuit in one arm of which said photoelectric cell islocated and the output terminals of which are electrically coupled tothe input terminals of said switching means; and wherein said switchingmeans is made conductive in response to the output of said bridgecirCuit when unbalanced and then causes said astable multivibrator tostart oscillating.
 4. An improvement as set forth in claim 1 whereinsaid second control means comprises a motor which is drivingly coupledto said variable exposure factor element.
 5. An improvement as set forthin claim 1 wherein said light quantity control means has manual controlsand said second control means comprises display means for guiding theuse of said manual controls.
 6. An improvement as set forth in claim 3wherein said switching means comprises a pair of switching circuitswhich are differential amplifier circuits.
 7. An improvement as setforth in claim 2 wherein said time constant circuits have a commonresistor network and wherein said switching means comprises a bridgecircuit one arm of which comprises said photoelectric cell, and a pairof switching circuits to which are applied the output of said bridgecircuit, and wherein said switching circuits are coupled to saidresistor network of said time constant circuits so as to complete saidtime constant circuits at one of two points, or not at all, according tothe state of said switching means, such point of completion being suchas to determine the sense of the disparity of said time constants.
 8. Animprovement as set forth in claim 3 wherein said astable multivibratorcomprises two transistors and a pair of time constant circuits whosecapacitors are coupled to a series of resistors differentially andunequally apportionable to said respective capacitors by selectionthrough said switching means of the circuit return point in said seriesof resistors.
 9. An improvement as set forth in claim 5 wherein saiddisplay circuit comprises a pair of lamps and a pair of diodes which areeach inserted in series with one of said lamps, but poled in theopposite direction relative to each other, the series lamp-diodecircuits being connected in parallel.
 10. An improvement as set forth inclaim 4 wherein said variable exposure factor element is a variableresistor and said light quantity control means comprises shutter speedcontrol means which comprises a CR time constant circuit comprising saidvariable resistor and a capacitor, a detector circuit for detecting avoltage drop across said capacitor, and electromagnet means coupled tosaid detector circuit for closing a shutter when said voltage drop hasreached a predetermined value, said motor being operatively coupled tosaid variable resistor for varying its resistance.